Magnet circuit control



Oct. 27, 1964 M. w. GRlFr-'Es 3,154,723

MAGNET CIRCUIT CONTROL Filed March 14 1961 i o 2&5/ l 2 21+ 56 y /4 tif/6 /7 f6 k :j *Q 34 26 Y Y A n 55 U13? i n 2 z -2' I 1 l 1 I hcw k y) I g 6j/4 7' 2 y) m l 40 l Z ME IN1/EN TOR.

United States Patent O 3,154,723 MAGNET CIRQUE@ CONTROL Milton W. Gritfes, Madison, Ohio, assigner to The Euclid Electric & Manufacturing Company, Madison, Ohio, a corporation of Ohio Filed Mar. 14, 1%1, Ser. No. 95,654 11 Claims. (Cl. 317-123) This invention relates to control circuits for electromagnets and more particularly to control circuits making possible the more eicient and rapid operation of electromagnets by substantially discharging the stored energy in the magnet before applying a full, unrestricted voltage in the reverse direction for removal of residual magnetism.

Electromagnets have been used for many years in the handling of ferrous and other magnetic materials, particularly in handling scrap material where other types of mechanical loading and transfer apparatus are rendered generally incapable of economical and otherwise satisfactory operation due, at least in part, to the varied shapes of many of the scrap pieces.

One of the diiiiculties attending the use of highly inductive devices of the type being considered resides in the fact that upon interruption of energizing current dow, the collapsing magnetic eld induces a potential in the device tending to maintain such current flow whereby the magnetic flux and magnetic attraction of the flux persists. This self-induced potential characteristically rises to high values, dependent upon the design of the energy absorbing means, resulting in destructive arcing effects upon the current interrupting means and high stresses upon the insulation of the magnet. Dissipation of this stored magnetic energy and the reversal of the magnet current to remove the residual magnetic held in the iron core accounts for considerable lost time in the discharge phase of magnet operation.

The longer the time required to discharge the magnet and to remove the residual magnetism from the magnet the longer must the operator allow the magnet to dwell at the point of discharge. Small pieces of metal, the last to fall free from the magnet, will be scattered if the magnet is moved prematurely from the discharge point. Rapid energy discharge and removal of residual magnetism in order to drop all size pieces quickly and bunched together is an obvious advantage to be desired in most magnet handling operations.

Many circuits have been devised in the past for attempting to eliminate or reduce the decay period of industrial magnets so that materials will drop cleanly from the magnet with only a short delay following interruption of the energizing current. Generally these prior art devices require the magnet, upon disconnection from the energizing source, to be reconnected immediately, with reversed connections through some current limiting resistor means. Other types of existing systems or devices apply a reversed current to the magnet before it is disconnected from the energizing source so that an overlapping continuous discharge circuit is provided back through the energizing and generator circuit. An extremely undesirable feature of these prior attempts to solve the problem of discharging residual self-induced potential from the magnet was the fact that comparatively high potentials were created during the discharge operation, which caused severe arcing in the current interrupting device and necessitated frequent replacement of the lift switch contacts.

It is a principal object of this invention to provide a circuit for controlling the operation of electromagnets in a manner providing for more rapid and efficient operation than has heretofore been possible.

A further object of this invention is to provide a circuit for controlling the operation of an electromagnet in which substantially complete discharge of the stored energy in 3,154,723 Patented Oct. 27, 1964 the magnet takes place through a discharge circuit having improved means for more rapidly removing the energy.

Another object of this invention is to provide a circuit for controlling the operation of an electromagnet in which a reversed current is applied to the magnet only after the discharge current of the magnet has been reduced to substantially zero value.

Another object of this invention is to provide a circuit for controlling the operation of an electromagnet in which full line potential is applied to the magnet for causing reversed current to rise more rapidly in the magnet to remove the residual magnetic energy therefrom more quickly.

An additional object of this invention is to provide a permanently connected circuit for controlling the operation of an electromagnet in which the potential across the energizing or lift contacts and the electromagnet at the time of interruption is limited to lower values than has heretofore been possible with conventional discharge means.

Other objects and advantages of this invention will be in part obvious and in part explained by a reference to the accompanying specification and drawings.

In the drawings:

FIGURE 1 diagrammatically illustrates a control circuit according tothe present invention;

FIGURE 2 illustrates a switch suitable for use in the present control circuit; and

FIGURE 3 is a graph showing the nature of the magnet potential and current as functions of time.

Generally, the present control circuit is one in which the electro-magnet is provided with a permanently connected discharge path which includes resistor means having the property of increasing in resistance as the applied potential decreases. This resistor is referred to as a nonlinear resistor whose ohmic value is an inverse function of applied potential raised to some exponential power.

The relationship of voltage V and current I in the nonlinear resistor may be expressed as V=Cl1/n Where C is a constant, and n an exponent which for the present purposes is preferably approximately 4 or 5. Further, additional circuitry including automatic interrupting means is coupled to the electromagnet to control the application of reverse current thereto. The said additional circuit or that controlling the application of the reverse current to the magnet, includes switch means and is responsive to the self-induced potential in the magnet, the reverse current being applied only after the discharge current from the magnet flowing through the non-linear resistor has been reduced to a very low value substantially equal to zero.

Referring to FIGURE 1 of the drawings a pair of conductors 1t? and 11 are connected to a direct current source to energize the system, with line 10 positive and line 11 of negative polarity, as indicated. An electromagnet for lifting metallic pieces is shown at 12 and includes a winding 13 of high inductance. Energization of magnet winding 13 to perform a lifting operation is obtained by moving master switch 14 to lift position. Contact 15 is closed, completing a circuit from positive line 1@ through a coil 16, a blocking rectifier 17, closed contacts 13 of a drop contactor 27 to negative line 11. Energization of coil 16 of the contactor 19 actuates the armature 19a thereof to close contacts 2l) and 21 and open interlock contacts 22. Power from line 10 flows through contact 2), the winding 13 of electromagnet 12, contact 21, to negative line 11 to energize the magnet. The contacts 22 are part of a second circuit which will be described in detail later in the specification. It is apparent that the closing of lift contacts 2t) and 21 creates a complete circuit between the positive and negative lines 1t) and 11 through the winding 13 of electromagnet 12. Once this circuit is closed, the magnet is energized and a magnetic flux field is created by the magnet for the lifting operation.

A circuit is permanently connected in parallel with magnet l2 to dissipate energy stored in the magnet upon interruption'of the energizing circuit. This parallel discharge circuit is indicated generally by the numeral 23 and includes a non-linear resistor 24, rectifier means 25 and series coil 26 of a drop contactor 27. The blocking rectifier 25 passes conventional current in the direction of the arrow only. This is in a discharge direction for the winding 13 and the flow of current through the resistor during the lift operation is prevented. Since the collapsing magnetic field of a magnet induces a potential thereacross which is reverse to the applied potential, the rectifier means 25 being in series with resistor 24 provides only for the flow of current through the circuit 23 during such collapsing field.

The non-linear resistor 24 used as an energy absorbing means has the ability to increase its ohmic resistance value as the potential applied thereto decreases. The relation between current and potential drop across the resistor is given by the formula I=KVn where I=cur rent, V=voltage, n=exponent having a numerical value of preferably 4 to 5 and K is a constant. It is apparent that the effective resistance value is a function of the potential applied, raised to at least an exponential value of 4.

Since power absorbed by the resistor 24 during the discharge of the magnet is the effective product of current squared times resistance at any instance, the rate of power absorption is greatly increased by using a non-linear resistor of this type as compared with a linear type.

One type of non-linear resistor is made from a fused silicon carbide and ceramic mixture and is obtainable in a variety of forms. For example, one series of material is manufactured and sold by the General Electric Co. and is called Thyrite If desired, other types of nonlinear resistors may be used such as one consisting of a pile of carbon discs magnetically compressed so as to vary the total resistance as an exponential function of the potential. This type of resistor is also very effective in decreasing the discharge time as compared with a linear resistor.

The preferred non-linear resistor type is the static form not requiring electro-mechanical means of changing its value in relation to the potential applied.

At the end of a lifting operation and after the switch contacts Ztl and 21 are open, the collapse of the eld in magnet 12 results in creation of a self-induced potential causing the polarity of the winding 13 of magnet 12 to be reversed. The potential produced at this instant of circuit interruption will be dependent upon the effective resistance of the discharge path circuit I3 around the magnet. In consideration of the characteristically high induced voltage stresses on insulation between magnet windings and leads, it is desirable to keep the maximum potential approximately 2 to 21/2 times line voltage. This potential is determined by the resistance characteristics of resistor 24.

As previously mentioned, one of the important aspects of the present control circuit is that of providing means preventing the application of a reverse potential to the magnet until such time as the self-induced potential is reduced to a predetermined level, generally, this level is nearly zero. This result is obtained by providing circuit interrupting means which are coupled to the electromagnet l2 to control in time and magnitude the application of the reverse current flow. This circuit interrupting means includes switch means and is responsive to the combined self-induced potential of the electromagnet and discharge current in the external discharge path 23. Part of the response is in maintaining the switch means open until such time as the self-induced potential in the magnet is reduced to the predetermined level.

Specifically, referring once again to FIGURE l of the drawings, the switch contacts 22 control the operation of the circuit interrupting and switch means just mentioned. The remainder of this control circuit includes solenoid 2S which is connected across resistor 24 by master switch contact 29 and interlocking contact 30 of switch 27. Series windings 26 and 31 are wound on a common magnetic core and are connected into the magnet discharge circuit and reverse current energizing circuitry, respectively. The winding of solenoid 28 produces a magnetic pull on one section 32 of an armature that is countered by a magnetic attraction of the opposite portion of the armature produced by discharge current flowing through winding 26. The armature is pivoted about a point intermediate to section 32 and another section 33 and is capable of movement to bring either portion of the armature in contact with the solenoid core ends as magnetic forces are produced by the several windings. The armatures 32 and 33 are shown in FIGURES l and 2 in the deenergized position with contacts 34 and 35 of drop contactor 27 open. The effective magnetic force produced by current flowing in winding 26 is much greater by virtue of the close proximity of one portion 33 of the armature in its rest position than the magnetic attractive force produced by winding Z3 through a large air gap. The magnetic attraction of winding 28 becomes effective to operate contactor 27, however, onlyV after the discharge current through series winding 26 has decayed to a relatively low value releasing the magnetic hold on its portion of the armature. From above it can be seen that simultaneous energization of solenoids 26 and 28 produces countering forces on armature sections 32 and 33. The contactor 27 remains inoperative until the discharge current from the magnet flowing through winding 26 has nearly decayed to a zero value. At this time the magnetic attraction produced in winding 23 due to the discharge potential drop across resistor 24 is sufficient to operate contactor 27 and close contacts 34 and 35. Note that the magnet discharge potential is effectively high at the same time that the magnet current discharge value is nearly zero. This phenomenon is due largely to the non-linear characteristics of the resistor 24. A linear resistor causes both potential and current to decay together so that the circuit is relatively ineffective without the use of anonlinear type resistor as previously described. A secondary action will now take place since the contactor 27 has been closed to apply reverse line potential to magnet i2. While armature section 32 is held magnetically against the core of winding 28 a build up in reverse magnet current flow through contacts 34 and 35 will sufficiently excite series winding 31 to finally cause the armature to move again to its initial rest position and open contacts 34 and 35. The amount of reverse current required in winding 3l to over-power the attractive force of winding 28 is determined by the amount of resistance presented by variable resistor 36 that is inserted into the circuit of solenoid 28 by the opening of contacts 30. The value of resistor 36 is set by the operator as a means of arbitrarily determining the amount of reverse current through the magnet to obtain clean drop of light weight scrap.

The operation of the circuit is as follows: the switch contacts 2t) and 2l are closed by operation of the energizing switch 14 thereby completing the energizing circuit from conductor 1t) to conductor 11 through the lifting magnet l2. A magnetic field is produced by the electromagnet to perform the lifting operation and the magnet is moved to the appropriate position for discharge. After the material is properly positioned for unloading, the switch contacts 2t) and 2f are opened by deenergizing solenoid 16 thereby interrupting the supply of current to the electromagnet and enabling collapse and decay of the magnetic field in the electromagnet.

Once the decay period is started, the material begins to drop free of the magnet with the smaller pieces hanging on due to the residual magnetic flux in the magnet. Simultaneous with collapse of the eld a self-induced potential arises within the magnet and this self-induced potential produces a current passing serially through solenoid 26, rectifier and through the non-linear resistor 24 contained in the shunt discharge circuit 23 as well as solenoid 28 which is connected in parallel with resistor 24. The non-linear resistor, as already mentioned, causes the field to decay more rapidly than would be the case if a linear type resistor were used producing the same initial potential. The discharge current and self-induced potential applied to solenoids 26 and 28, respectively, cause each to generate a magnetic field, however, due to the fact that essentially the same strength field is present initially in each of these members, the mechanical bias in favor of solenoid 26 present in the switch, armatures 32 and 33 preclude it from making any movement which would act to close the reverse current contacts 34 and 35.

As the magnet field of the electromagnet continues to decay, the field within solenoid 26 decreases more rapidly than that in solenoid 23, for reasons already explained. This then enables the solenoid 28 to overcome the advantage in the switch armature means 33 and the switch contacts 34 and 35 close. It should be noted that the switch contacts 34 and 3S are not closed until the discharge current in the magnet I2 is nearly reduced to zero. Closing of the switch contacts 34 and 35 places magnet I2 across the line in a reverse direction and starts the reverse current flow through the winding of the electromagnet to demagnetize the same and cornpletely remove any remaining iiux from the magnet. Maximum rate of rise of reverse current is obtained by direct connection of the line potential without current limiting resistors in the circuit. Closure of contacts 34 and places winding 3l serially in the reverse current circuit. As the reverse magnet current flowing through the solenoid 31 increases to a predetermined value, the attractive force of solenoid 28, as explained hereinabove, is overcome causing the switch armature. to be pulled against winding 31 core. The point at which the attraction of solenoid 28 becomes too small to hold the switch contacts 34 and 3S closed can be readily and easily adjusted by changing the resistance of the variable resistor 36.

Referring to FIGURE 2 of the drawings there is shown one form of switch and switch operator means which contains the solenoids 26, 31 and 2S together with the armatures 32 and 33. The various parts have been numbered to coincide with the corresponding parts in FIG- URE l. In this drawing it can readily be seen that simultaneous application of a discharge potential from magnet 12. above line potential to solenoid 28 and the discharge current through series winding 26 will retain the switch armatures 32 and 33 in an unmoved position, the unmoved position being one in which the switch contacts 34 and 3S remain open. After most of the energy of the magnet has been expended in the non-linear resistor 24 and the potential across the terminals of the magnet reduced to somewhat above line potential, the magnetomotive force produced by solenoid 25 is overcome by the attraction of the field generated by solenoid 2S. When this action occurs the armature then moves from the position indicated, to one where the contacts 34 and 35 are closed. The final step of the operation, of course, is that wherein the bucking solenoid 31 overcomes the attraction of solenoid 28 and the mechanical advantage of the armature section 32 permits it to move to the open position, that is, the position shown in the drawing.

FIGURE 3 of the drawing shows the relationship between the magnet current and the magnet potential with respect to the same time axis just before and through the discharge cycle. The line 37 represents the full magnet excitation current Value and the line 38 the full line potential. The peak potential is represented by the numeral 39 and this peak occurs at the instant of interruption of the switch contacts 20 and 21. This peak is of opposite polarity to the line potential. Rapid magnet current decay through the non-linear resistor 24 to a Value of nearly zero is indicated by the point 4t) on the potential curve. It is at this point that the reverse current switch contacts 34 and 3S close to apply potential of reverse polarity to the magnet.

The reverse polarity from negative supply line 11 is now connected directly to the magnet. Maximum rate of rise of reverse current is obtained by direct connection of the negative supply line without any current limiting resistors in the circuit. This feature permits reduction of any stored energy in the magnet field to a minimum value in the shortest possible time. At the instant the reverse current contacts 34 and 35 open, a sudden rise in potential of opposite polarity is produced across the terminals of the magnet, again by the collapse of the magnetic field as shown by the numeral 4I on the potential curve of FIGURE 3. This value of potential is not high enough to cause distress of the switch contacts 34 and 35 even without a shunt discharge path, however, an auxiliary discharge path may be provided if desired. Since the magnet energy is proportional to current squared and the reverse current usually never exceeds more than 15% of the full magnet excitation there is relatively little difficulty in dissipating the energy the moment of opening of switches 34 and 35.

It is apparent that the present invention provides a control circuit for electromagnets which provides for rapid decay of the residual flux or self-induced energy from the magnet at the end of a lifting operation. The arrangement of the circuit operation is also such a nature that minimum replacement of switch contacts and other operating parts is obtained.

While this invention has been described with respect to a specific embodiment thereof, it will be apparent to those skilled in the art that numerous modifications and alterations may be made without departing from the spirit or scope of the invention.

What is claimed is:

l. A circuit for controlling the operation of an electromagnet comprising, first circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet for creating a magnetic field providing for a lifting operation, a shunt circuit connected to said electromagnet and including a resistor element connected in series with rectifier means poled to provide for dissipation of energy stored in said magnet upon interruption of the energizing current flow at the end of a lifting operation, and second circuit interrupting means coupled to said electromagnet to control the application of a reverse current fiow thereto, said second circuit interrupting means including switch means and being responsive to the self-induced potential in said electromagnet to maintain said switch means open, preventing the application of the reverse current to said magnet until the self-induced energy is reduced to a predetermined level.

2. A circuit for controlling the operation of an electromagnet comprising first circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet for creating a magnetic field providing for a lifting operation, a shunt circuit connected to said electromagnet and including a resistor having the property of automatically increasing its resistance as the potential applied thereto decreases, rectifier means connected in series with said resistor means, said resistor means and said rectifier means cooperating to provide for dissipation of stored energy in said electromagnet after interruption of the energizing current flow at the end of a lifting operation, and second circuit interrupting means coupled to said electromagnet to control the application of reverse current flow thereto, said second circuit interrupting means including switch means in circuit with said electromagnet and being responsive to the self-induced potential in said electromagnet to maintain said switch means open, preventing the application of the reverse current to said magnet until thev stored energy is reduced to a predetermined level.

3, A circuit for controlling the operation of an electromagnet comprising, first circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet for creating a magnetic field providing for a lifting operation, a shunt circuit connected to said electromagnet and including resistor means having the property of automatically increasing its resistance as the potential applied thereto decreases, rectifier means connected in series with said resistor means in a manner preventing the flow of current through said resistor means by an applied potential during the lifting operation, and second circuit interrupting means coupled to said electromagnet to control the application of the reverse current flow thereto from said source, said second circuit interrupting means including switch means and being responsive to the self-induced potential in said electromagnet to maintain said switch means open, preventing the application of the reverse current to said magnet until the self-induced energy is reduced to an approximate zero value.

4. A circuit for controlling the operation of an electromagnet comprising means for controlling energizing current flow from a direct current source through said electromagnet for creating a magnetic eld providing for a lifting operation, a shunt circuit connected to said electromagnet and including a resistor element connected in series with rectifier means to provide for dissipation of energy stored in the field of said magnet upon interruption of the energizing current flow at the end of the lifting operation, switch means connected in said control circuit providing for application of a reverse current to said electromagnet, a solenoid and means for coupling the same to said electromagnet, said solenoid being responsive to the self-induced potential of said electromagnet to operate said switch means and apply a reverse current to said electromagnet when said self-induced energy is reduced to a predetermined level, and a separate solenoid operably coupled to said electromagnet to interrupt said reverse current flow when the intensity thereof reaches a predetermined value and demagnetizes said electromagnet.

5. In a circuit for controlling the operation of an electromagnet, said circuit including interrupting means for controlling energizing current iiow from a direct current source through said electromagnet for creating a magnetic eld providing for a lifting operation, the combination comprising a shunt circuit connected to said electromagnet and including a non-linear resistor having the property of automatically increasing its resistance as the potential applied thereto decreases, said resistor providl ing for dissipation of energy stored in said magnet on interruption of the energizing current flow at the end of a lifting operation, switch means connected in said control circuit providing for application of a reverse current to said electromagnet, a solenoid coupled to said electromagnet and being responsive to the self-induced potential of said electromagnet to operate said switch means and apply reverse current to said electromagnet when said self-induced potential is reduced to a predetermined level.v

6. A circuit for controlling the operation of an electromagnet, said circuit including interrupting means for controlling energizing current iioW from a direct current source through said electromagnet for creating a magnetic iield providing for a lifting operation, the combination comprising a shunt circuit connected to said electromagnet and including a non-linear resistor having the property of automatically increasing its resistance as the potential applied thereto decreases, rectifier means connected in series with said resistor means, said resistor means and said rectifier means cooperating to provide for dissipation of energy stored in the lield of said electromagnet at the interruption of the energizing current iiow at the end of Va lifting operation, switch means connected in said control circuit providing for application of a reverse current to said electromagnet, a solenoid coupled to said electromagnet and being responsive to the self-induced potential of said electromagnet to operate said switch means and apply a reverse current to said electromagnet when said self-induced potential is reduced to a predetermined level, and separate solenoid means operably coupled to said electromagnet to interrupt said reverse current flow when the self-induced potential is reduced to a predetermined level.

Y7. A circuit for controlling the operation of an electromagnet comprising, first circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet in a direction creating a magnetic field providing for a lifting operation, a shunt circuit connected to said electromagnet and including a non-linear resistor having the property of inherently increasing its resistance as the'potential applied thereto decreases, rectifier means connected in series With said resistor, said resistor and said rectifier means cooperating to provide for discharge of self-induced potential generated in said electromagnet by interruption of the energizing current ow at the end of a lifting operation, switch means connected in said control circuit providing for application of a reverse current to said magnet, first and second solenoids connected for simultaneous energization by the self-induced potential in said electromagnet, said first and second solenoids being respectively operatively coupled to said switch means in a manner causing .Said switch means to be biased toward an open position, nonlinear resistor means connected in series with one of said solenoids causing the energizing current flowing therethrough to decay more rapidly than the energizing current flowing through the other of said solenoids, and a solenoid separate from said first and second solenoids operatively connected to said electromagnet to interrupt said reverse current ow when `the residual energy is reduced to a predetermined level,

8. A circuit for controlling the operation of an electromagnet comprising, rst circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet in a first direction creating a magnetic iield providing for a lifting operation, a shunt circuit connected to said electromagnet and including a resistor element connected in series with rectifier means to provide for discharge of stored energy in said magnet upon interruption of the energizing current iiow at the end of a lifting operation, switch means connected in said control circuit providing for application of a current in an opposite direction to said electromagnet, a circuit in parallel with said electromagnet and including a first solenoid means effective to bias said switch means to an open, non-operative position when fully energized, a non-linear resistor connected in series with said iirst solenoid means and providing increasing resistance asV the applied potential decreases, second solenoid means connected in parallel with said non-linear resistor and being effective to bias said switch means -to a closed position, and solenoid means separate from said first and second solenoid means selectively connected to said electromagnet to interrupt said reverse current iiow when the stored energy in .said electromagnet is reduced to a predetermined level.

9. In a circuit for controlling the operation of an electromagnet combination comprising, first circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet in a first direction creating a magneticV ield providing for a lifting operation, a shunt circuit connected to said electromagnet and including resistor means having the property of automatically increasing its resistance as the potential applied thereto decreases, rectier means connected in .series with said resistor means in a manner preventing the flow of energizing current through said resistor means during the lifting operation, switch means connected in said control circuit providing for application of a reverse current to said electromagnet, first solenoid means connected in .said shunt circuit and being eiective to bias said switch means to an open, non-operative position when fully energized, a non-linear resistor connected in series with said rst solenoid means to provide increasing resistance as the applied potential decreases, second solenoid means being connected in parallel with .said non-linear resistor to bias said switch means to a closed operation position.

10. A circuit for controlling the operation of an electromagnet comprising, rst circuit interrupting means for controlling energizing current flow from a direct current source through said electromagnet in a direction creating a magnetic eld providing for a lifting operation, a shunt circuit connected to said electromagnet and including a resistor having the property of automatically increasing its resistance as the potential applied thereto decreases, rectifier means connected in series with said resistor means, said resistor means and said rectier means cooperating to provide for discharge of self-induced potential generated in said electromagnet by interruption of the energizing current flow at the end of a lifting operation, switch means connected in said control circuit providing for application of a reverse current to said electromagnet, first solenoid means connected in said shunt circuit and being eiTective to bias said switch means to an open, non-operative position when fully energized, a nonlinear resistor connected in series with said first solenoid means to provide increasing resistance as the applied potential decreases, second solenoid means being connected in parallel with said non-linear resistor to bias said switch means to a closed operating position when the potential across said first solenoid means falls below a pre-determined level, and a solenoid separate from said first and second solenoids operatively connected to be energized by the reverse current iiowing in said electromagnet and disposed to counter the magnetic effect of said second solenoid to interrupt .said reverse current ow when the reverse current in said electromagnet is reduced to a predetermined level.

ll. In a circuit for controlling the operation of an electromagnet the combination comprising, switch means connected in said control circuit providing for the application of a deenergizing current to said electromagnet, rst solenoid means connected in parallel with said electromagnet to bias said switch means to an open, non-operative position when fully energized, a non-linear resistor having the property of automatically increasing its resistance as the applied potential decreases connected in series with said rst solenoid means and second solenoid means connected in parallel with said non-linear resistor and being effective to bias said switch means to a closed, operating position.

References Cited by the Examiner UNITED STATES PATENTS 2,181,539 11/39 Wertz 317-123 SAMUEL BERNSTEIN, Primary Examiner. WALTER L. CARLSON, Examiner. 

10. A CIRCUIT FOR CONTROLLING THE OPERATION OF AN ELECTROMAGNET COMPRISING, FIRST CIRCUIT INTERRUPTING MEANS FOR CONTROLLING ENERGIZING CURRENT FLOW FROM A DIRECT CURRENT SOURCE THROUGH SAID ELECTROMAGNET IN A DIRECTION CREATING A MAGNETIC FIELD PROVIDING FOR A LIFTING OPERATION, A SHUNT CIRCUIT CONNECTED TO SAID ELECTROMAGNET AND INCLUDING A RESISTOR HAVING THE PROPERTY OF AUTOMATICALLY INCREASING ITS RESISTANCE AS THE POTENTIAL APPLIED THERETO DECREASES, RECTIFIER MEANS CONNECTED IN SERIES WITH SAID RESISTOR MEANS, SAID RESISTOR MEANS AND SAID RECTIFIER MEANS COOPERATING TO PROVIDE FOR DISCHARGE OF SELF-INDUCED POTENTIAL GENERATED IN SAID ELECTROMAGNET BY INTERRUPTION OF THE ENERGIZING CURRENT FLOW AT THE END OF A LIFTING OPERATION, SWITCH MEANS CONNECTED IN SAID CONTROL CIRCUIT PROVIDING FOR APPLICATION OF A REVERSE CURRENT TO SAID ELECTROMAGNET, FIRST SOLENOID MEANS CONNECTED IN SAID SHUNT CIRCUIT AND BEING EFFECTIVE TO BIAS SAID SWITCH MEANS TO AN OPEN, NON-OPERATIVE POSITION WHEN FULLY ENERGIZED, A NONLINEAR RESISTOR CONNECTED IN SERIES WITH SAID FIRST SOLENOID MEANS TO PROVIDE INCREASING RESISTANCE AS THE APPLIED POTENTIAL DECREASES, SECOND SOLENOID MEANS BEING CONNECTED IN PARALLEL WITH SAID NON-LINEAR RESISTOR TO BIAS SAID SWITCH MEANS TO A CLOSED OPERATING POSITION WHEN THE POTENTIAL ACROSS SAID FIRST SOLENOID MEANS FALLS BELOW A PRE-DETERMINED LEVEL, AND A SOLENOID SEPARATE FROM SAID FIRST AND SECOND SOLENOIDS OPERATIVELY CONNECTED TO BE ENERGIZED BY THE REVERSE CURRENT FLOWING IN SAID ELECTROMAGNET AND DISPOSED TO COUNTER THE MAGNETIC EFFECT OF SAID SECOND SOLENOID TO INTERRUPT SAID REVERSE CURRENT FLOW WHEN THE REVERSE CURRENT IN SAID ELECTROMAGNET IS REDUCED TO A PREDETERMINED LEVEL. 